Method for forming a soft nitride layer in a metal surface

ABSTRACT

Improved processes for nitriding and carbonitriding steel alloys by the use of gas mixtures containing a small amount of air or oxygen. In the case of nitriding, a metal specimen is held in an atmosphere of ammonia gas to which is added 0.5 to 5 percent air by volume or 0.1 to 1 percent oxygen by volume at temperatures between 450* and 650*C. As a result of the treatment, a soft nitride layer is formed in the surface of the specimen. When carbonitriding is performed, a gas mixture containing 30 percent ammonia gas, 30 percent carbon monoxide, 5 to 10 percent air or 1 to 2 percent oxygen and the remaining volume percentage nitrogen, is employed. In this case, also a soft nitride layer substantially free from Fe2N is formed in the metal surface. According to this invention, the time required to perform a desired nitriding or carbonitriding is greatly reduced, while the wear resistance, fatigue resistance and toughness of the metal are highly improved.

United States Patent [1 1 Kondo et al.

[111 3,748,195 [451 July 24,1973

[ METHOD FOR FORMING A SOFT NITRIDE LAYER IN A METAL SURFACE [73] Assignee: Nissan Motor Company, Limited,

Kanagawa-ku, Japan 221 Filed: June 11, 1911 21 Appl. No.: 152,242

[30] Foreign Application Priority Data July 21, 1970 Japan 45/63848 Aug. 31, 1970 Japan 45/76202 [52] US. Cl. 148/165, 148/166 [51] Int. Cl. C23c 11/16, C23c 11/18 [58] Field of Search.................... 148/16, 16.5, 16.6, 148/167, 20.3

[56] Reierences Cited UNITED STATES PATENTS 3,519,257 7/1970 Winter et al 148/165 X 1,793,309 2/1931 Egan 148/166 1309,6159 5/1933 Egan 148/166 OTHER PUBLICATIONS Metals Handbook, Vol. 2 8th Ed. pgs. 119-428 &

Nitrocycle Process Helps Solve Deep Well Problems" reprinted from The Petroleum Engineer, March 1954, pg. 98 & 99.

Primary Examiner--Charles N. Lovell A2tarneyRobert E. Burns and Emmanuel J. Lobato [57] ABSTRACT improved processes for nitriding and carbonitriding steel alloys by the use of gas mixtures containing a small amount of air or oxygen. In the case of nitriding, a metal specimen is held in an atmosphere of ammonia gas to which is added 0.5 to 5 percent air by volume or 0.1 to 1 percent oxygen by volume at temperatures between 450 and 650C. As a result of the treatment, a soft nitride layer is formed in the surface of the specimen. When carbonitriding is performed, a gas mixture containing 30 percent ammonia gas, 30 percent carbon monoxide, 5 to 10 percent air or 1 to 2 percent oxygen and the remaining volume percentage nitrogen, is employed. In this case, also a soft nitride layer substantially free from Fe,N is formed in the metal surface. According to this invention, the time required to perform a desired nitriding or carbonitriding is greatly reduced, while the wear resistance, fatigue resistance and toughness of the metal are highly improved.

6 Claims, 18 Drawing Figures Pmmmm 3748;195-

SIEEI 1 0F 7 DIFFUSED NITROGEN AMOUNT a :3 L6 /L/ (in U a 53 COMPOUND Li LAYER g Q OEPT |2 a .g-J- 8 o x a: E r IO 5 5 3 (U5 8 g F- O g O v t I |O AMOUNT OF AIR ADDED (percent by volume) PATENTED NITRIDE LAYER DEPTH (mm) SHEEI 2 BF 7 TREATING TIME (Hours) PATENTED SMEIEUFY NITROGEN COMPOUND LAYER O CARBON 5 AMOUNT 0 AIR ADDED (Percent by volume) DEPTH m 2 0 6 l. I. O O O O owwnta zwoomtz .6 E302,

PATENIEBJULMISH SHEEI 8 (IF 7 SUR- 12 FACE DEPTH BELOW BELOW SUREGE (microns) FACE DEPTH SURHSGZ (microns) 9p SUR DEPTH BELOW FACE SURF/ CE (microns) FeKoL CKoL BELOW SURFACE (microns) JTCARBON RICH LAYER DEPTH SUR- FACE PMENYEDJULZA I875 SHELU 7 Iii 7 Fly /5 METHOD FOR FORMING A SOFT NITRIDE LAYER IN A METAL SURFACE This invention relates to improved processes of nitriding and carbonitriding metals such as steels by the use of a gas mixture containing a small amount of elemental oxygen containing gas such as air or pure oxygen.

In the conventional ammonia gas nitriding process, metals to be treated such as steels, titanium or the like function as catalysts to facilitate dissociation of ammonia into hydrogen and nitrogen. Nitrogen penetrates and diffuses into the surface of the metals to form a nitride layer in the surface. However, due to the fact that the nitriding atmosphere has a low concentration of nitrogen, the metals are required to be processed at approximately 500C for as long as 40 to 60 hours. Furthermore, a hard and brittle compound layer of forroferri mononitride (Fe,N) which has the disadvantage of peeling off, tends to be formed on the nitride layer as a result of the treatment. Still further, the metals treatable according to the nitriding process cover only the so-called nitridable steels containing Al, Cr, Ti, V, Mn, Si, etc. as alloying elements.

Typical of the carbonitriding processes which are carried out at relatively low temperatures ranging from 450 to 600C is one using a KCN containing salt bath agent. However, due to the use of toxic potassium cyanide (KCN) this process includes dangerous operations and great care must be exercised to prevent environmental pollution. Another known carbonitriding process is one employing a gas mixture of carbon monoxide and ammonia gas, the carbon monoxide being obtained by incomplete burning of charcoal. In the gas process, metals to be treated are held in the carbonitriding atmosphere at temperatures of 550 to 570C for approximately L to 2 hours. However, it has been found that this process is considerably inferior to the salt bath process in many respects such as compound layer depth, wear resistance and fatigue properties.

It is therefore an object of this invention to provide improved processes of nitriding and carbonitriding with a view to overcome the above-said disadvantages.

It is another object of this invention to provide improved processes of nitriding and carbonitriding in which a small amount of air or oxygen is introduced into the nitriding or carbonitriding atmosphere to thereby reduce the time required to complete the treatmeat.

It is a further object of this invention to provide improved processes of nitriding and carbonitriding which improve the wear resistance, fatigue properties and toughness of steel alloys.

It is still further object of this invention to provide improved processes of nitriding and carbonitriding which are well suited to mass production.

in the accompanying drawings:

FIG. 1 is a plot showing the amount of nitrogen diffused and the depth of a formed iron-nitrogen compound layer against the amount of air introduced;

FIG. 2 is a plot showing the diffusion depth of nitrogen against treating time, obtained when metal specimens are treated by the conventional process and the present process;

FIGS. 3A and 3B are X-ray diffraction patterns of the surface layers of the metal specimens nitrided by the conventional process and the present process;

FIGS. 4 through 8 are micrographs taken at a magni fication of 400 showing the cross-sectional microstructure of the metal specimens nitrided by the conventional process and the present process;

FIG. 9 is a plot showing the amounts of nitrogen and carbon diffused into the surface of a metal and the depth of a iron-nitrogen compound layer against the amount of air introduced according to this invention;

FIGS. 10 through 13 are analytical results obtained by an Electron Probe of an electrolytic iron after treatment by the conventional process and the present process; and

FIGS. 14 through 17 are micrographs taken at a magnification of 400 showing the crosssectional microstructure of the metal specimens carbonitrided by the conventional process and the present process.

ln accordance with one embodiment of this invention, a small amount of air or elemental oxygen is introduced into the nitriding atmosphere of ammonia gas in order to facilitate dissociation of the gas to increase the nitrogen concentration of the atmosphere, so that the nitriding time is reduded. By so doing, a soft nitrided layer is formed in the surface of the metals being treated, while the ferroferi mononitride (Fe N) is pre vented from being produced in the surface. The terms iron-nitrogen compound and iron-nitrogen compound layers as referred to herein refer to the iron nitridecontaining complex and layers thereof characterized by being substantially free from this Fe,N nitride. It is to be noted that this process can be carried out more satisfactorily by applying a coating of sodium silicate or s0- dium silicate-refractory clay mixture on the inner wall of the furnace used for treatment, usually, made of stainless steel and also on the surfaces of the instruments or holders for holding the metals, followed by drying the coating.

FIG. 1 shows how the amount of nitrogen diffused and the depth of a iron-nitrogen compound layer are influenced by the increase in the amount of air introduced. These values were obtained when sheets of mild steel containing 0.07 percent carbon and having a thickness of 0.1 mm were held in the ammonia-air mixtures at 570C for two hours. It is evident that the amount ofdiffused nitrogen and the iron-nitrogen compound layer depth rapidly increase as the amount of air is increased from zero to 0.5 percent by volume, but that they both exhibit a small rate of increase with a further increase in the amount of air to 5 percent. The iron-nitrogen compound layer depth appears to reach its maximum with an introduction in the neighborhood of 5 percent air by volume. It has been found that an undesirable oxidation tends to take place as the amount of air is increased further to above 5 percent. Thus, it is important that the amount of air added be in the range of 0.5 to 5 percent, and preferably, 1 to 3 percent by volume. In the case where elemental oxygen is added into the atmosphere instead of air, the appropriate amount is 0.l to l percent, and preferably, 0.2 to 0.6 percent by volume. The experiments conducted by the Inventor have shown that it is desirable to bring the temperature of the nitriding atmosphere to between 450 and 650C.

FIG. 2 is a plot of nitrogen difusion depth against holding time, obtained when the metal specimens of electrolytic iron containing 0.008 percent carbon were treated by the conventional ammonia gas nitriding process and the present process. The amount of air added according to this invention was approximately I percent by volume. In the plot, the solid lines represent the nitrogen diffusion depths reached at three different temperatures by the present process, and the broken lines those obtained by the conventional process. FIG. 2 shows the superiority of the present process over the conventional process in that the time required to obtain a desired nitrogen diffusion depth is greatly reduced.

FIGS. 3A and 3B show, for comparison, X-ray diffraction patterns of the surface layers of the metal specimens nitrided by the conventional process and the present process, respectively. In the case of the conventional process, the test specimens were held in an atm o' sphere of I percent ammonia at 550C for 2 hours, while in the present process one percent air by volume was added to the ammonia gas atmosphere. A comparison between the patterns indicates clearly that no such a peak as represents in FIG. 3A the presence of Fe,N which tends to peel off can be found in FIG. 3B, which means that the present process is effective in preventing the formation of such a compound layer.

FIGS. 4 through 8 are cross-sectional micrographs taken at a magnification of 400 showing the microstructures of the metal specimens nitrided under different treating conditions. In the micrographs, A indicates the iron-nitrogen compound layer, 8 represents the layer into which nitrogen is diffused, and C the base metal or core. FIG. 4 shows a mild steel nitrided by the conventional process, that is, in an atmosphere of 100 percent ammonia gas at 570C for 2 hours; FIG. 5 shows that treated in an atmosphere of 99 percent ammonia gas and 1 percent air at 570C for 2 hours; FIG. 6 represents that treated in an atmosphere of 95 percent ammonia gas and 5 percent air at 570C for 2 hours; FIG. 7 represents SCH, treated in an atmosphere of 99 percent ammonia gas and l percent air at 600C for 90 minutes; and FIG. 8 represents cemented Scr treated in an atmosphere of 99 percent ammonia gas and 1 percent air at 570C for 2 hours. (All percentages are by volume.) SUI-I3 and Ser 20 are steel alloys, which are defined by the Japan Industrial Standards.

Given in Table I are the results of the simple bending fatigue test conducted in conformity to the 4 kg-m Shenk type fatigue test with the test specimens nitrided by the Tufftriding process and the present processv layer (z-phase) and a micron diffusion layer rcsulted. The hardness of the diffusion layer was mHv 946 and therefore the metal processed according to the present process was found to exhibit an excellent wear resistance.

As an example of nitriding Scr20, an article of a carburized Scr20 steel was processed in an atmosphere of 99 percent ammonia gas and 1 percent air at 570C for 2 hours. An 6 -phase iron-nitrogen compound layer l5 microns thick was obtained. This experiment indicated that a combination of the cementation and the nitriding decreased the hardness gradient from the surface to the inside of the article, resulting in an increase in the strength to resist peeling.

From the above, it will be appreciated that the present process can be applied to all steel alloys. Further more, it will be understood that according to this invention the time required to obtain a desired soft nitride layer is greatly reduced, while the wear resistance, fatigue resistance and toughness of the metals are highly improved.

In accordance with another embodiment of this invention, a small amount of air or elemental oxygen is introduced into a carbonitriding atmosphere containing carbon monoxide and ammonia gas. It has been found desirable that the amount of air added is 5 to ID percent by volume (or i to 2 percent oxygen by volume) when the amounts of carbon monoxide and ammonia gas are both 30 percent by volume. The remaining volume percentage is nitrogen. Care must be taken not to increase the amount of carbon monoxide above 30 percent since sooting would otherwise take place. The amount of ammonia gas also should not be increased further from the economical point of view since the amount of nitrogen diffusion reaches its maximum at this 30 percent by volume concentration. It is important that the amount of air added be below 10 percent since an undersirable oxidation would otherwise take place. The temperature of the carbonitriding atmosphere is desirably selected to be between 450 and 600C. As in the case of nitriding, it is to be noted that this process can be carried out more satisfactorily by applying a coating of sodium silicate or sodium silicate-refractory clay mixture on the inner wall of the treating furnace and also on the surfaces of the holding instruments or holders, followed by drying the coating.

TABLE 1 Fatigue limit, wpa. (kg/mm?) Before After Magnitude Treating treattreat of increase mortal Treating method temperature/time nmnt mcnt (percent) r A s C. J0 2' .2 ML!) 120.? 825V mmulmld air. i iildnii. ii ii li f f 25,2 421.0 120.

2H. 'J 02. I) ilii. 2 244M, llullliilllitl material g t wfi tfii xd g V 62." 1m 2 It is seen from Table I that the present process can improve the fatigue property to the same extent as the Tufftriding process.

Experiments were conducted to confirm that favorable results are obtainable when the present process is applied to SCH3 and Scr20.

FIG. 9 shows how the amounts of nitrogen and carbon and the depth of iron-nitrogen compound layer are influenced by the increase in the amount of air introduced into the carbonitriding atmosphere. These values were obtained when sheets of mild steel containing 0.068 percent carbon and having a thickness of 0.1 mm were treated at 570C for minutes. A combustion analysis and a distillation analysis were carried out to determine the amounts of carbon and nitrogen diffused, respectively. It is evident that both the amount of diffused nitrogen and the iron-nitrogen compound layer depth increase with an increase in the amount of air introduced and the iron-nitrogen compound layer depth reaches its maximum in the neighborhood of 5 percent air by volume. Furthermore, it has been found that an undersirable oxidation tends to take place as the amount of air is increased above 10 percent. Further, the amount of carbon diffused slightly decreases with an increase in the amount of air. Thus, it is preferable that the amount of air introduced is between S and 10 percent by volume. In the case where elemental oxygen is added into the treating atmosphere instead of air, the appropriate amount is l to 2 percent by volume.

FIGS. 10 through 13 show the results obtained by an Electron Probe microanalyzer on the test specimens of ania and FIG. 17 represents the results obtained with S 45C after treatment under the same conditions as in the case of FIG. 16. These two micrographs indicate that according to this invention the 5 25C and S 45C can be processed in a shorter time than by the conventional process to reach the same end result. The thicknesses of the soft nitride layers were l and 9 microns for the S 25C and S 45C, respectively, and the depth of the diffusion layer was 0.l mm for the S 25C. 8 25C and S 45C are steel alloys with are defined by the Japanese Industrial Standards.

Given in Table II are the results of the simple bending fatigue test conducted in conformity to the 4 kg-m Fatigue limit, wpa. (kg/1pm.

electrolytic iron containing 0.008 percent carbon after treatment by the conventional process and the present process. From FIG. I0 which shows the test specimen processed in an atmosphere of 30 percent carbon monoxide, 30 percent ammonia and the remaining volume percentage nitrogen at 570C for 90 minutes, it is evident that when the carbonitriding atmosphere contains no air nor oxygen the nitrogen rich layer is relatively shallow. In contrast, when the carbonitriding atmosphere contains even a small amount of air, for example, 5 percent air by volume according to this invention, a deep nitrogen-rich layer is obtained as is shown in FIG. ll. FIGS. 12 and 13 show the results of the analyses of carbon in the test specimens after treatment under the same conditions as in the cases of FIGS. 10 and II, respectively, that is, according to the conventional process and to the present process, respectively. A comparison between FIGS. 12 and 13 indicates that when air is introduced, the depth of the carbon-rich layer increases in contrast to the slight decrease in the amount of carbon diffused upon introduction of air in increasing amounts (shown in FIG. 9).

FIGS. 14 through 17 are micrographs taken at magnification of 400 showing the cross-sectional microstructures of metal specimens carbonitrided under different conditions. In the micrographs, A indicates the ironnitrogen compound layer, 13 represents the diffusion layer and C the base metal or core. The X-ray diffraction analysis conducted by the Inventor has indicated that the surface compound layer obtained at temperatures from 450 to 600C by the present process is an Fe,N phase soft nitride layer. FIG. 14 shows the electrolytic iron (0.008% C) carbonitrided under the same conditions as in the case of FIG. 10, that is, according to the conventional process; and FIG. represents that after treatment under the same conditions as in the case of FIG. 11, that is, according to this invention. A comparison between FIGS. 14 and 15 indicates clearly that by adding air or oxygen into the carbonitriding atmosphere it is possible to increase the depth of soft nitride layer. FIG. I6 shows the results obtained with S C after treatment in an atmosphere of percent carbon monoxide, 30 percent ammonia gas, 5 percent air and the remaining nitrogen at 570C for 90 minutes;

Shenlt type fatigue test with the test specimens treated by the Tufftriding process and the present process.

It is seen from Table II that the present process can improve fatigue resistance to the same extent as the Tufftriding process.

From the foregoing, it will be appreciated that according to this invention the treating time is greatly reduced, while a sofi nitrided layer substantially free from the Fe,N, nitride is formed in the surfaces of the metals to be treated with resulting increases in the wear resistance, fatigue resistance and toughness of the metals.

What is claimed is:

l. The method for treating iron and steel articles to form compound-nitride layers thereon, said layers being characterized by the substantial absence of the Fe,N nitride, which comprises the steps of introducing said article into a furnace, providing said furnace with a catalytic surface comprising sodium silicate, maintaining said furnace at a temperature range 450650C., filling said furnace with a treating atmosphere of a gas selected from the group of nitriding and carbonitriding gaseous atmospheres and adding to said atmosphere up to 1 percent oxygen equivalent by volume of an elemental oxygen-containing gas, maintaining said article in said furnace at said temperature for a period sufficient to form said compound layer thereon.

2. A method according to claim I wherein said compound-nitride layer is a nitride layer and said gaseous atmosphere is a nitriding gas which contains ammonia in gaseous form and said oxygen-containing gas consists of 0.5 to 5 percent by volume of said atmosphere of air.

3. A method according to claim I in which said gaseous atmosphere consists of a nitriding gas which contains ammonia in gaseous form and said oxygencontaining gas is 0.1 to l percent elemental oxygen by volume of said atmosphere.

4. A method according to claim 1 in which said compound nitride layer comprises a carbonitride layer and in which said gaseous atmosphere consists essentially of up to 30 percent carbon monoxide by volume, up to 30 percent ammonia gas by volume, and the balance of nigen in an amount of 1-2 percent by volume of said atmosphere.

6. The method according to claim 1 wherein said catalytic surface is a sodium silicate-refractory clay mixture applied to at least one portion of the walls of said furnace.

i t i i It 

2. A method according to claim 1 wherein said compound-nitride layer is a nitride layer and said gaseous atmosphere is a nitriding gas which contains ammonia in gaseous form and said oxygen-containing gas consists of 0.5 to 5 percent by volume of said atmosphere of air.
 3. A method according to claim 1 in which said gaseous atmosphere consists of a nitriding gas which contains ammonia in gaseous form and said oxygen-containing gas is 0.1 to 1 percent elemental oxygen by volume of said atmosphere.
 4. A method according to claim 1 in which said compound nitride layer comprises a carbonitride layer and in which said gaseous atmosphere consists essentially of up to 30 percent carbon monoxide by volume, up to 30 percent ammonia gas by volume, and the balance of nitrogen and to which is added air in an amount of 5 to 10 percent by volume of said atmosphere.
 5. A method according to claim 1 in which said atmosphere is a carbonitriding atmosphere consisting substantially of up to 30 percent carbon monoxide by volume, up to 30 percent ammonia gas by volume, and the balance nitrogen and to which is added elemental oxygen in an amount of 1-2 percent by volume of said atmosphere.
 6. The method according to claim 1 wherein said catalytic surface is a sodium silicate-refractory clay mixture applied to at least one portion of the walls of said furnace. 